Bitmap Font Color

OpenGL OpenGL: Basic Coding
1

Hello,

I currently have a program that prints to the screen using the bitmap font technique. On the bitmap the background is black and the characters are white. Because of this I can enable blending, call glBlendFunc(GL_ONE, GL_ONE) and do a glColor4f(…) to print in any color I choose. This works fine.

The problem arises when I add a background texture and then try to print to the screen. I can no longer print in the color I want. For example, I add the background texture, then do a glColor4f(1.0f,0.0f,0.0f,1.0f) (since I want red text) then do a call to my print function. The text prints, however, it’s not red, it’s more a light red, a pinkish red if you will. I’m not sure if the problem has to do with my texture environment or my blending function.

Here’s some code:

[b]
bool Font::RenderScene()
{
    glEnable(GL_TEXTURE_2D);
    glTexEnvf(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_REPLACE);
    DrawQuad(-1.0f,-1.0f,1.0f,1.0f,textures[0]); //background texture

      glEnable(GL_BLEND);
      glBlendFunc(GL_ONE, GL_ONE);
    glColor4f(1.0f, 0.0f, 0.0f, 1.0f); //red text
    glTexEnvf(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_MODULATE);
    arial->Print(-0.9f, 0.6f, "Hello World!");

    return true;
}
 [/b]
[/b][/QUOTE]

again, the color of the text works if i don't add the background texture. however, when i do add the background texture, the color is always lighter than its suppose to be. Any ideas? Thank in advance.

Regards,
JQ
2

put glDisable(GL_TEXTURE_2D); just before drawing the text.

3

Disable textures? The text itself is a texture. The characters are just parts of a bitmap image.

4

Shouldn’t your blending function be something like
glBlendFunc(GL_SRC_COLOR, GL_ONE_MINUS_SRC_COLOR)
?

5

I’ve tried this blend function, however its not quite what i want. The letters are the color i want, however the background for the texture (black) appears, and I don’t want that. Which is why I was using GL_ONE, GL_ONE. Can you think of anything else I can try? Oh, I am using OpenGL ES 1.1.

Regards,
JQ

6

Well you did not tell us everything :slight_smile:

If you have alpha you have to take it in account. Try with glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

7

Well originally my bitmap font image was a simple RGB image, no alpha. However, after mindless time wasting I realized that I could just use imagemagik and make the black background transparent:

convert oldimage -transparent black newimage

With the background now gone I can simply use transparency…and Viola!

So your right, I didn’t tell you everything, but thats because I was not even taking alpha into account.

Thanks -NiCo- and ZbuffeR for your help.

Regards,
JQ